1. Field of the Invention
This invention relates generally to energy transfer devices and methods and, more particularly, to devices and processes for transcutaneous energy transfer (TET) to a secondary coil implanted in a subject.
2. Related Art
Many medical devices are now designed to be implanted in humans or animals, including pacemakers, defibrillators, circulatory assist devices, cardiac replacement devices such as artificial hearts, cochler implants, neuromuscular simulators, biosensors, and the like. Since many of these devices require a source of power, inductively coupled transcutaneous energy transfer (TET) systems are coming into increasing use. A TET system may be employed to supplement, replace, or charge an implanted power source, such as a rechargeable battery. Unlike other types of power transfer systems, TET systems have an advantage of being able to provide power to the implanted electrical and/or mechanical device, or recharge the internal power source, without puncturing the skin. Thus, possibilities of infection are reduced and comfort and convenience are increased.
TET devices include an external primary coil and an implanted secondary coil, separated by intervening layers of tissue. The two coils constitute a transcutaneous transformer. The transformer is designed to induce alternating current in the subcutaneous secondary coil, typically for transformation to direct current to power the implanted device. TET devices therefore also typically include an oscillator and other electrical circuits for periodically providing appropriate alternating current to the primary coil. These circuits, referred to for convenience herein as "TET primary circuits," receive their power from an external power source.
Generally, the non-implanted portions of conventional TET systems are attached externally to the patient, typically by a belt or other fastener or garment, such that the primary coil of the TET is operationally aligned with the implanted secondary coil. The TET primary circuits and external power supply are also generally attached to the patient's body at or near the site of the attachment of the primary coil. Such a configuration typically is disadvantageous, however, particularly when the patient is sleeping or resting. For example, if a patient is sleeping on a mattress, the patient would likely be uncomfortable, or restricted in movement, if all or some of the TET primary circuits and external power supply were attached to the patient. In addition to discomfort or restriction of movement, additional disadvantages of such body attachments include possibilities of injury to the patient or the devices. Movements of the patient may alter the position of the primary coil so that it is not properly positioned over the implanted secondary coil to achieve a desired or required transfer of power.
To overcome these drawbacks, other conventional approaches require only the primary coil be attached to the patient. Wires connect the primary coil to the TET primary circuits, which, with the power supply, may be located at a distance from the patient outside of the sleeping or resting surface. However, such an alternative configuration also has significant disadvantages. First, the primary coil is still attached to the patient and therefore subject to the above drawbacks may cause discomfort or restriction of movement. Also, as the patient moves, the wires connecting the externally attached primary coil to the TET primary circuits may become tangled or entangled with bedding or the patient. In addition to being uncomfortable, such tangling may result in dislodging the primary coil from its required alignment; it may injure the patient, such as by restricting blood or oxygen supply; or it may interfere with tubes or other devices attached to the patient.